We propose to determine the total DNA sequence of several ribosomal cistrons (rrn) of Escherichia coli. Ribosomes play such a central role in gene expression that it is important to understand their structure and function in detail. Thus the significance of this work derives primarily from the importance of having complete and accurate primary sequence data for all of the ribosomal RNAs of E. coli. Already, the sequence of the 16S rDNA has been valuable, in conjunction with other data, in speculations about possible secondary and tertiary structure in 30S ribosomal subunits. In addition to structure-function consideration, evidence indicates that the rrn cistrons of E. coli are not identical. This heterogeneity may have functional significance. Precise nucleic acid sequence data is required to evaluate any detected heterogeneity. Careful comparisons of heterogeneity in rrn cistrons could change basic ideas of how ribosomes function in the overall process of gene expression.